K.J. Coughlan

PLOT-SCALE RAINFALL-RUNOFF CHARACTERISTICS AND MODELING AT SIX SITES IN AUSTRALIA AND SOUTHEAST ASIA

During major runoff events when most soil loss occurs, runoff is likely to dominate the rainfall-driven erosion processes. Thus accurate estimation of the runoff rate is critical to soil loss predictions. At plot scale, the Green-Ampt infiltration model is commonly assumed to be able to describe the temporal variation of the infiltration rate over a storm event. Field measurements of both rainfall intensity and runoff rate at 1-min intervals at six sites in the tropical and subtropical regions of Australia and Southeast Asia, however, strongly suggest that the apparent infiltration rate is closely related to the rainfall intensity and it is essentially independent of the cumulative infiltration amount, features not accord with the Green-Ampt infiltration equation. Furthermore, the storage effect and runoff rate attenuation are not negligible at the plot scale. With an initial infiltration amount to determine when runoff begins, an exponential distribution to describe the spatial variation in the maximum infiltration rate and a linear storage formulation to model the lag between runoff and rainfall, we were able to develop a satisfactory three-parameter model for the runoff rate at 1-min intervals within a storm event.

Methodology for a multi-country study of soil erosion management

Abstract This paper describes the theoretical framework used in interpreting data on runoff and soil loss from field experiments to yield information on soil erodibility. This theory has been employed in the form of computer programs in the field experiments in various tropical countries and Australia which have collaborated in the Australian Centre for International Agricultural Research Project 8551 entitled “The Management of Soil Erosion for Sustained Crop Production”. The paper also describes common features of the experimental methodology employed in this project, including a description of the set of data management programs employed. These programs are used to retrieve electronically logged data, to field-check, summarise and compile these data in a form suitable for the analysis programs employed. Subsequent papers in this series illustrate application of the theoretical and experimental methodology outlined in this paper.

Alley cropping for managing soil erosion of hilly lands in the Philippines

Abstract A field experiment was conducted on a hillslope to test three soil conservation-oriented alley cropping treatments, and to compare them with farmers practice in terms of their effects on soil erosion and runoff. The alley cropping treatments had 1-m-wide, leguminous shrub hedgerows (Desmanthus virgatus) established along the contour, with an alley width of 5 m. Besides the effect of hedgerows, the effects of mulching and minimum tillage were also studied. The farmers practice is characterised by up-and-down slope tillage operations and weed-free culture. Maize and mungbean were used as wet and dry season crops, respectively. Twelve erosion plots were laid out, with four treatments replicated three times. Each plot had a dimension of 6 m (across slope) by 12 m (down-slope), and was equipped with electronic data loggers that measured rates of rainfall and runoff as a function of time. Total soil loss, divided into bed load and suspended load, was measured after each rainfall event. Due to high rainfall amount, lack of soil protection and steep slope, soil erosion rates in the farmers practice were very large, reaching 100 to 200 t ha−1 year−1. However, these values were markedly reduced to a rate of less than 5 t ha−1 year−1 in the alley cropping treatments. The reduction of soil loss in the alley cropping was attributed to reduction in both total runoff volume and sediment concentration. A significant decrease in sediment concentration resulted from higher contact cover that effectively protects the soil from both rainfall detachment and runoff entrainment, and the reduction in overland flow velocity associated with mulching with plant residues and the presence of densely planted hedgerows. The reduction of total runoff was due to increased infiltration rates resulting from contour ploughing, terrace formation and high infiltration rates in soil adjacent to hedgerows.

Toward a Framework for Runoff and Soil Loss Prediction using GUEST Technology

In recent years, a number of physically based models have been developed for soil loss predictions. GUEST is one such model based on fundamental physical principles and the current understanding of water erosion processes. GUEST is mainly used to determine a soil erodibility parameter. To apply the model in a predictive mode, the model is simplified in a physically meaningful manner for flow-driven erosion processes, and 2 essential hydrologic variables are identified, namely total runoff amount and an effective runoff rate. These variables are required to determine soil loss for individual runoff events. A simple water balance model was developed and used to predict runoff amount from rainfall amount. The efficiency of this runoff amount model in prediction was over 90% using field data. A 1-parameter regression model (r2 ~ 0·9) for the effective runoff rate was also established which uses peak rainfall intensity in addition to rainfall and runoff amounts. The prediction of peak rainfall intensity for a given rainfall amount and storm type was also sought. The field data were from Goomboorian, near Gympie, in south-east Queensland and these data were used to test and validate both models. Results overall are satisfactory and the approach adopted is promising. A framework for soil loss prediction is established within which individual parts can be further refined and improved.

Soil erosion processes in sloping land in the east coast of Peninsular Malaysia

Understanding soil erosion processes is essential in appreciating the extent and causes of soil erosion and in planning soil conservation. A comprehensive study was conducted on steep slopes on the east coast of Peninsular Malaysia where the rainfall pattern is monsoonal. Rates of rainfall and runoff, and soil loss, were accurately measured from four large plots of 1000 m2 and a bare plot of 20 m2. Instruments used include Parshall flumes, height recorders, a pluviometer and a runoff tipping bucket recording on a datalogger, with the associated computer hardware and software for data processing and analysis. Because of the slope of the land (about 18%) and the existence of large well-defined flow pathways, soil was largely eroded through the processes of entrainment and reentrainment. Soil loss and runoff were particularly high where the pathways were very pronounced. Other factors influencing soil erosion were disturbance to the soil surface, which produced easily entrainable material, and percent contact cover. The parameter β, calculated for a large number of events, was found to vary according to the availability of entrainable material. There was a gradual general decrease of β with time although most values were in the range 0.37 to 0.25. Suspended load was commonly more than 50% of total soil loss. Consequently, high chemical enrichment ratios were obtained. This has important implications on the reduction of soil quality through nutrient and organic matter losses.

A validation test of WEPP to predict runoff and soil loss from a pineapple farm on a sandy soil in subtropical Queensland, Australia

Monthly runoff and soil loss simulated by WEPP (Water Erosion Prediction Project) were compared with field observations on a pineapple farm in south-east Queensland for a 3-year period. The soil at the site is sandy. Slope length and steepness are 36m and 5.5%, respectively. Three treatments, namely bare, farmers’ conventional practice, and mulching of the furrows, were used. Infiltration and erodibility parameters were determined using WEPP-recommended equations and measurable soil properties. These parameters were also calibrated using the runoff and soil loss data for the bare plot only. Apart from the soil loss prediction for the mulching treatment, for which WEPP did not perform well, the average coefficient of efficiency in runoff and soil loss predictions was –0.02 using soil property-based parameter values and 0.66 using calibrated parameter values. The corresponding r 2 values are 0.57 and 0.81, respectively. On the whole, WEPP is able to reproduce the trend and variations in runoff and soil loss among different treatments for the site. Parameter values based on measurable soil properties would greatly under-estimate the runoff and soil loss for the site. Thus, use of WEPP outside its US database requires calibration with locally obtained data. It was also found that WEPP does not seem to model effectively the situation where there is considerable flow impediment with the furrows covered with mulch. We are unable to reject WEPP because the statistical performance indicators are reasonable for the site, and because the model is so complex that it is nearly impossible to pinpoint the source of discrepancy and articulate the model deficiency on physical grounds.

Erodibility evaluation and the effect of land management practices on soil erosion from steep slopes in Leyte, the Philippines

The paper reports selected results from an extensive experiment in which runoff and soil loss were measured from a range of treatments applied to runoff plots at the Visayas State College of Agriculture (VISCA) in the Philippines. Treatments included bare soil, common agricultural practice, and a range of soil-conserving practices, including intercrops and hedgerows of Leucaena leucocephala. The presence of a maize crop reduced soil loss but not runoff relative to the bare plot. Addition of hedgerows approximately halved runoff from the steep experimental plots, thereby reducing soil loss, though sediment concentration was similar to the cropped treatment. The addition of an intercrop of peanut further reduced both runoff and sediment concentration in the runoff, thus leading to a substantial reduction in soil loss. The erodibility of a nearby different soil at low slope (10%) was found to be high, but its erodibility appeared to be increased following cultivation.

A NOTE ON SAMPLING ERRORS IN THE RAINFALL AND RUNOFF DATA COLLECTED USING TIPPING BUCKET TECHNOLOGY

Data precision for rainfall intensity and runoff rate using tipping bucket technology is controlled by the bucket volume, V, the catchment area, A, and the sampling interval, t. The maximum absolute error is less than V(A.t)–1 and the standard error is given by V(v _ 6 A.t)–1, both of which are independent of the magnitude of rainfall intensity and runoff rate being measured. Therefore, the relative error can be quite large during events with low rainfall intensity or runoff rate. The magnitude of the sampling error given above can be used to assist in experiment and equipment design and in data analysis.

Soil and nutrient loss under rozelle (Hibiscus subdariffa L. var. altissima) at Khon Kaen, Thailand

Rozelle (Hibiscus subdariffa L. var. altissima) was grown in hydrologically defined plots at Khon Kaen, Thailand (latitude 16°30′N, longitude 102°50′E, altitude 195 m). The loss of water, soil and plant nutrients from these plots under different types of soil management was measured for the three growing seasons from 1989 to 1991. Plots were 30 m long, 5 m wide, and slope 3.6%, and the soil was a loamy sand with 5% clay and 79% sand. Treatments included up and downslope cultivation, contour cultivation with and without subsoiling, and no tillage. A bare plot was also installed to measure soil erodibility parameters. Runoff and soil loss from treatment plots was greatest from the up and downslope cultivation treatment, though not exceeding 20% of rainfall or 4 tonnes/ha respectively in any growing season. Despite the modest loss of soil by water erosion, the average enrichment ratio for nitrogen was 5.5. Thus the loss of nitrogen would be important on this sandy soil, especially as rozelle is commonly grown in this region with little or no fertilizer input. Soil, water and nutrient loss was also measured for a bare soil plot. Soil loss from this plot was about ten times higher than for the most erosive treatment. Most eroded sediment was transported from the bare plot in broad, shallow rills in which the sediment concentration was close to the transport limit, which was less than the concentration produced by rainfall detachment and re-detachment because of the low slope of the plots, and the high detachability of this soil. Of the four management treatments investigated, contour cultivation emerged as the most practicable. Subsoiling was ineffective in further reducing runoff and soil loss, and if no cultivation was carried out, plant yield was reduced, even though this treatment was most effective in reducing soil erosion.

Griffith University Erosion System Template (GUEST)

Excluding gully processes and mass movement, the rate of erosion of bare soil depends on the rate of overland flow and rainfall, on the erodibility and depositability of surface soil, and on the features of filling if this occurs. The program GUEST is designed to analyse data collected from runoff plots of simple form’, and to yield an approximate non-dimensional erodibility parameter denoted by β. The parameter β has a theoretical basis, and is more physically meaningful if flow-driven erosion processes dominate those due to rainfall impact.